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A high-gain and low-noise low noise amplifier (LNA) includes a
differential amplifier, a pre-amplifier and an impedance matching
network. The differential amplifier includes a first input end and a
second input end coupled to a grounded impedance. The pre-amplifier
includes an input end and an output end. The impedance matching network
is coupled between the first input end of the differential amplifier and
the output end of the pre-amplifier for matching an input impedance of
the differential amplifier with an output impedance of the pre-amplifier.
The present invention provides a LNA structure with low noise, high gain
and easy design.

1. A high-gain and low-noise low noise amplifier (LNA) comprising: a
differential amplifier comprising a first input end and a second input
end coupled to a grounded impedance; a pre-amplifier comprising an input
end and an output end; and an impedance matching network coupled between
the first input end of the differential amplifier and the output end of
the pre-amplifier for matching an input impedance of the differential
amplifier with an output impedance of the pre-amplifier.

2. The high-gain LNA of claim 1 further comprising an input impedance
coupled to the input end of the pre-amplifier.

3. The high-gain LNA of claim 1 wherein the pre-amplifier is a
common-source single transistor amplifier further comprising: an
impedance-matching load connected to a drain of the transistor; and a
degenerate impedance coupled between the source of the transistor and
ground.

4. The high-gain LNA of claim 1 wherein the second input end of the
differential amplifier is coupled to a capacitance impedance.

5. The high-gain LNA of claim 1 wherein the differential amplifier has two
differential output ends.

Description

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a low noise amplifier, and more
particularly, to a low-noise and high-gain low noise amplifier.

[0003] 2. Description of the Prior Art

[0004] With the widespread usage of cellular phones, mobile communication
has become an integral part of daily life. Many design companies endeavor
to improve every circuit block of the communication system. Low noise
amplifiers (LNAs) belong to the receiver part of a communication system,
with the function to enlarge received signals and to suppress the
receiver's noise.

[0005] Commonly, LNA structure is based on a single-input-to-single-output
design. In this structure the input end of the mixer that follows the LNA
has to be single-ended as well. This design has limited ability to reduce
the common mode noise of the mixer and the signal leaked from the
oscillator to the mixer. Applying a differential output structure to the
LNA can solve the problem. The most simple and common way to achieve a
LNA with a differential output is by designing a
differential-input-to-differential-output structure. This structure
requires an extra transformer to convert a single-ended signal received
at the antenna to a differential signal at the output end. This
transformer not only adds extra cost to the capital, but its power loss
also increases the NF (Noise Figure) of the entire receiver and encumbers
system performance. Therefore, the preferred design for a receiver is a
LNA with a single-input-to-differential-output structure.

[0006] Please refer to FIG. 1. FIG. 1 is a diagram of a prior art LNA 10
based on a single-input-to-differential-output design. The LNA 10
includes a transformer 12 and a differential amplifier 14. The
transformer 12 is a passive single-input-to-differential-output
transformer formed by the metal coils on the integrated circuit. The
differential amplifier 14 comprises a differential pair of transistors M2
and M3, and an output impedance Z.sub.L for matching the output impedance
seen into RFout. The transformer 12 is coupled to the differential
amplifier 14 to amplify the high frequency signal entering at the input
end RFin. The metal coils of the transformer 12 occupy large areas of the
circuit layout and add to manufacturing costs.

[0007] Please refer to FIG. 2. FIG. 2 is a diagram of a prior art LNA 20
based on a single-input-to-differential-output design. The LNA 20
includes a first-end input impedance 21, a second-end input impedance 22
and a differential amplifier 24. The differential amplifier 24 comprises
a differential pair of transistors M2 and M3, and an output matching
impedance Z.sub.L. The gate of one of the differential pair transistors
is coupled to ground through the second-end input impedance 22. The gate
of the other transistor is coupled to the input end RFin through the
first-end input impedance 21. The prior art LNA structure 20 is
advantageous over the prior art LNA structure 10 in that it removes metal
coils serving as transformers, saves more space for other circuitry and
reduces manufacturing costs. When a differential amplifier is operated at
high frequencies, however, the current source Is used to bias the
differential pair cannot be viewed as an ideal high impedance current
source. Thus, when designing for the noise and gain for the prior art LNA
20, the transistor M2 cannot be treated as a common-source structure.
Therefore the prior art LNA 20 requires complicated impedance matching
designed at both input end and output end.

[0008] If a metal coil transformer is used to achieve a
single-input-to-differential-output LNA structure, large areas on the
circuit will be occupied, raising manufacturing costs. On the other hand,
if a single-input-to-differential-output LNA structure is achieved by
grounding one input end of the LNA, as demonstrated in prior art LNA 20,
the high frequency impact of the current source on the differential
transistors has to be taken into consideration. This high frequency
characteristic of a non-ideal current source increases the complexity
when designing the noise and gain for the LNA.

SUMMARY OF INVENTION

[0009] It is therefore a primary objective of the claimed invention to
provide a low noise amplifier (LNA) with high gain and low noise
performance and a related method to design the LNA.

[0010] Briefly described, the claimed invention discloses a high-gain low
noise amplifier comprising a differential amplifier, a pre-amplifier and
an impedance matching network. The differential amplifier comprises a
first input end and a second input end coupled to a grounded impedance.
The pre-amplifier comprises an input end and an output end. The impedance
matching network is coupled between the first input end of the
differential amplifier and the output end of the pre-amplifier for
matching an input impedance of the differential amplifier with an output
impedance of the pre-amplifier.

[0011] It is an advantage of the present invention that the LNA has low NF
(Noise Figure), high power gain and is easy to design when compared to
prior arts.

[0012] These and other objectives of the present invention will no doubt
become obvious to those of ordinary skill in the art after reading the
following detailed description of the preferred embodiment that is
illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a diagram of a prior art
single-input-to-differential-output LNA

[0014] FIG. 2 is a diagram of another prior art
single-input-to-differential-output LNA

[0015] FIG. 3 is a diagram of a first embodiment of a high-gain LNA of the
present invention.

[0016] FIG. 4 is a diagram of a second embodiment of a high-gain LNA of
the present invention.

[0017] FIG. 5 is a diagram of a third embodiment of a high-gain LNA of the
present invention.

[0018] FIG. 6 is a diagram of a fourth embodiment of a high-gain LNA of
the present invention.

DETAILED DESCRIPTION

[0019] Please address to FIG. 3. FIG. 3 is a diagram of a high-gain LNA 30
according to a first exemplary embodiment of the present invention. The
LNA 30 comprises a pre-amplifier 32, a differential amplifier 34, an
impedance matching network 36 and a grounded impedance 38. In this prior
art embodiment, the differential amplifier 34 includes a transistor M2
and a transistor M3 forming a differential pair. A load Z.sub.L is
connected between the power supply V.sub.DD and a drain of the transistor
M2 and a drain of the transistor M3, respectively. The drains of
transistor M2 and M3 represent the differential output ends of the LNA
30. The gates of the transistors M2 and M3 represent the differential
input ends of the differential amplifier 34. One input end of the
differential amplifier 34 (the gate of transistor M3) is coupled to
ground through the impedance 38. In this embodiment the impedance 38 is a
capacitor C.sub.B for isolating DC signals to the ground for the
transistor M3. The impedance matching network 36 is coupled between the
other input end of the differential amplifier 34 (the gate of the
transistor M4) and the output end of pre-amplifier 32.

[0020] Those skilled in the art know that when the frequency of a signal
is higher than a certain level, for example radio frequency, the
parasitics of the transistors in the circuit become major factors that
affect the entire system high frequency characteristics. At the same
time, the transmission of high frequency signals has to be considered in
view of electromagnetic waves to predict system performance more
accurately.

[0021] In the case of a high frequency system, the signal transmission
depends upon the impedance of related circuit blocks. When a high
frequency signal is transmitted from a circuit block to the next circuit
block with a different impedance, part of the signal is reflected. This
reduces the effectiveness of signal transmission at high frequencies. To
solve this problem, impedance matching must be taken into account when
designing for high frequency signal transmission.

[0022] In FIG. 3, the impedance matching network 36 is designed between
the pre-amplifier 32 and the differential amplifier 34 in order to match
the impedance of the two amplifiers. The pre-amplifier 32 is a
single-input-to-single-output amplifier, comprising a common-source
transistor M1 and a loaded matching impedance Z1. A source of the
transistor M1 is coupled to ground through a degeneration impedance
Z.sub.DEG. A drain of the transistor M1 is coupled to the loaded matching
impedance Z1. A gate of the transistor M1 is an input end of the
pre-amplifier 32. In the LNA 30 of present invention, an input signal
enters from one end of an input impedance Zin, and is amplified by the
pre-amplifier 32, as demonstrated in FIG. 3.

[0023] The purpose of the present invention is to achieve a high-gain LNA
with a single-input-to-differential-output structure. The pre-amplifier
32 is implemented to accomplish low noise design. According to Friis'
equation, the NF (Noise Figure) of the LNA 30 indicated in FIG. 3 is
decided by the NF of the transistor M1. Friis' equation is as follows:
F LNA .apprxeq. F 1 + F 2 - 1 G A .times. .times. 1

[0024] where F.sub.LNA is the NF of the LNA 30

[0025] F1 is the NF contributed by the transistor M1

[0026] F2 is the NF contributed by the transistor M2 and the transistor M3
of the differential amplifier 34

[0027] G.sub.A1 is the available power gain contributed by transistor M1

[0028] According to the equation, because of the available power gain
G.sub.A1 provided by the pre-amplifier 32, the impact of F2 on F.sub.LNA
becomes insignificant. The main contributor to the NF of the entire
system F.sub.LNA is the NF contributed by the pre-amplifier 32. Since the
pre-amplifier 32 is a single-transistor amplifier, its NF is smaller than
that of the differential amplifier 34. Thus, adding the pre-amplifier 32
not only improves the NF of the LNA 30, but also increases the available
power gain of the LNA 30. From the design point of view, pre-amplifier 32
simplifies the design of the LNA 30, since the transistor M1 is the only
factor to be considered during the optimization between the noise and
gain of the system.

[0029] Compared to the prior art LNA 10 with the
single-input-to-differential-output structure in FIG. 1, the present
invention does not require large chip area for the passive transformer,
and thus avoids the loss due to the passive transformer. According to
Friis' equation, the first level loss directly contributes to the NF of
the entire single-input-to-differential-output system. This loss is
especially severe on a high loss substrate (ex: silicon substrate).
Compared to the prior art LNA 20 with the
single-input-to-differential-output structure in FIG. 2, the present
invention has less noise and is easier to design. The complexity of
designing a ground point for the LNA 20 at high frequencies results in
the difficulty of matching both ends of the differential pair. Therefore
in a real integrated circuit, the node of current source and the sources
of the differential pair are not a virtual ground point. As a result, the
current source inevitably contributes additional noise to the entire LNA.

[0030] The best embodiment of the present invention is performed with
MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) devices, but
is not limited to MOSFET. The claimed invention can also be applied to
bipolar junction transistors (BJT) and other active devices with an
amplifying function. The transistor M1 in FIG. 3. can be replaced by a
BJT device or other active devices with an amplifying function. In the
following embodiments, the transistors M1, M2 and M3 corresponding to
FIG. 3 can be MOSFETs, BJTs or any active device with an amplifying
function and the exact device types are not exclusively mentioned.

[0031] Additionally, the differential pair comprised by the transistors M2
and M3 in FIG. 3 is not limited to a 2-transistor structure. The present
invention also includes a differential pair with a cascode structure.
Please refer to FIG. 4. FIG. 4 is a diagram of a second embodiment of a
high-gain LNA 40 of the present invention. The LNA 40 includes a
pre-amplifier 42, a differential amplifier 44, an impedance matching
network 46 and a grounded impedance 48. Compared to the LNA 30 in the
first embodiment of the present invention, the differential amplifier 44
comprises 4 cascode transistors, with the 2 extra transistors, M4 and M5
biased by VB1 and VB2, respectively. The transistors M4 and M5 increase
the power gain of the differential amplifier 44 and improve its stability
due to increased isolation.

[0032] Please refer to FIG. 5. FIG. 5 is a diagram of a third embodiment
of a high-gain LNA 50 of the present invention. The LNA 50 is extended
from the LNA 40. The LNA 50 includes a pre-amplifier 52, a differential
amplifier 54, an impedance matching network 56 and a grounded impedance
58. In the differential amplifier 54 of the LNA 50, sources of the
transistors M2 and M3 are in series connection to degeneration
inductances Z.sub.D2 and Z.sub.D3, respectively. The series connection
between the sources of the transistors M2 and M3 and the degeneration
inductances Z.sub.D2 and Z.sub.D3 betters the linearity of the
differential amplifier 54.

[0033] Please refer to FIG. 6. FIG. 6 is a diagram of a fourth embodiment
of a high-gain LNA 60 of the present invention. The LNA 60 has a
switching structure that provides 2 operating modes: high gain mode and
low gain mode. The LNA 60 includes a pre-amplifier 62, a differential
amplifier 64, an impedance matching network 66 and a grounded impedance
68. The differential amplifier 64 comprises 6 transistors M2 through M7.
Drains of the transistors M4 and M5 are coupled to V.sub.DD through an
impedance Z.sub.L1, separately. The drains of transistor M6 and M7 are
coupled to V.sub.DD through an impedance Z.sub.L2, separately. An
impedance Z.sub.G1 is coupled between drains of the transistors M4 and
M6, and an impedance Z.sub.G2 is coupled between drains of the
transistors M5 and M7. When the differential amplifier 64 operates under
high gain mode, the transistors M4 and M5 remain on and the transistors
M6 and M7 remain off. When a high frequency signal enters the
differential amplifier 64, one part of the signal travels through the
transistor M2, the transistor M4 and the impedance Z.sub.L1, and reaches
a positive end of RFout. The other part of the signal travels through the
transistor M3, the transistor M5 and the impedance Z.sub.L1, and reaches
a negative end of RFout. Thus, an output differential signal from the
original high frequency input signal is formed at both ends of RFout.
Similarly, when the differential amplifier 64 operates under low gain
mode, the transistors M6 and M7 remain on and the transistors M4 and M5
remain off. Therefore, the amplifier 64 serves as a passive network
comprised by the impedances Z.sub.L1, Z.sub.L2 and Z.sub.LG.

[0034] The prior art LNA 10 achieves a single-input-to-differential-output
structure by implementing a metal coil transformer. Manufacturing costs
and losses due to the passive transformer are two major drawbacks of this
prior art. Using a different approach, the prior art LNA 20 achieves a
single-input-to-differential-output structure by grounding one input end
of the LNA 20. This prior art requires impedance matching at both ends of
the differential pair transistors, and thus increases design complexity.
The present invention achieves a single-input-to-differential-output
structure by coupling one input end of the differential amplifier through
a grounded impedance and defining the other input end of the differential
amplifier from the pre-amplifier. In the present invention the main
contributor to the NF of the differential amplifier is the pre-amplifier
with a single-transistor structure. Thus the present invention has better
noise performance. The pre-amplifier also increases the total power gain
of the LNA in the present invention. And since the transistor M1 is the
only factor to be considered during the optimization between the noise
and gain of the entire system, it is simpler to design a LNA structure as
demonstrated in the present invention. The
single-input-to-differential-output structure of the high-gain and low
noise LNA in the present invention is implemented by inserting one
transistor in front of the differential pairs. Therefore the present
invention discloses an innovation in the circuit topology to achieve a
single-input-to-differential-output high-gain and low noise LNA. In
conclusion, the present invention has several advantages: low noise, high
gain and easy design.

[0035] Those skilled in the art will readily observe that numerous
modifications and alterations of the device and method may be made while
retaining the teachings of the invention. Accordingly, the above
disclosure should be construed as limited only by the metes and bounds of
the appended claims.